The importance of an interface between a human operator and a machine being used in a multimedia capacity or personal computer has recently increased. In order for an operator to comfortably and efficiently operate such a machine, it is necessary for the operator to briefly and instantly obtain a sufficient amount of information from the machine relating to the operation of the machine without an error. Because such information needs to be supplied to the operator by way of a display device, research has been made on various display devices to improve the quality thereof.
Currently, the demand for a small thin display device increases as the miniaturization of machines increases. For example, the further miniaturization of a lap-top personal computer comprising a display device integrated therewith, such as a "notebook" type personal computer surprisingly progresses, accompanied by significant technical revolution of a liquid crystal display serving as the display device.
At present, the liquid crystal display is widely used as an interface for various products including a lap-top personal computer, small televisions, watches and desk-top calculators.
Such a liquid crystal display (LCD) has been researched for making use of the advantages of a liquid crystal, such as a low driving voltage and low power consumption, and using as an interface between a human being and a machine and as a main display element in devices ranging from a small device to a large-capacity display device.
However, since this liquid crystal display does not emit light by itself, it requires a backlight and therefore electric power for driving the backlight. In fact, the backlight consumes more power than the liquid crystal. As a result, the operation of the backlight decreases the operating time of the battery or portable power supply and therefore places a limitation of the use of liquid crystal displays.
The liquid crystal display also has the shortcomings in that, since the liquid crystal display has a narrow angle of visibility, it is unsuitable for a large display device. Unfortunately, the displaying method employs the orientation state of liquid crystal molecules and the contrast changes with the angle even within the angle of visibility.
In consideration of a driving method for a visual display, although an active matrix method exhibits a sufficient response speed for handling a moving image, an active matrix method uses a TFT driving circuit, and it is thus difficult to increase the screen size due to pixel defects. The use of the TFT driving circuit is also expensive.
Although a simple matrix method as another driving method is lower in cost than an active matrix method, and makes it relatively easy to increase the screen size, the simple matrix method has shortcomings with respect to an insufficient response speed for handling a moving image.
On the other hand, a plasma display device, an inorganic electroluminescent device, an organic electroluminescent display, and the like are studied as luminescent display device.
The plasma display device employs plasma light emission in a low-pressure gas for display. Plasma display devices are suitable for increasing the size and capacity. However, the plasma display has the shortcomings with respect to providing a thin display panel and cost. Further, the plasma display device requires a high-voltage AC bias for driving and is thus unsuitable for a portable device.
The inorganic electroluminescent device is a green light emitting display or the like. The inorganic electroluminescent device is driven with an AC bias and requires several hundred volts for driving. It also is difficult to display a full-color image using available inorganic luminescent devices.
On the other hand, electroluminescence by an organic compound has been researched for a long period of time since strong fluorescent luminescence from an anthracene single crystal was observed by injecting a carrier in the early 1960s. However, the research has been made as fundamental research for injecting a carrier into an organic material because of low luminance, monochrome and a single crystal.
Since Tang et al. of Eastman Kodak Corp. reported an organic thin-film electroluminescent element in 1987 which permitted low-voltage driving and high-luminance luminescence, and which had a thin film structure comprising an amorphous luminescent layer, research and development have been extensively made on emission of light of red (R), green (G) and blue (B) primary colors, stability, increases in luminance, the thin film structure, manufacturing methods, etc.
Further, various new materials are invented on the basis of molecular designs, which are characteristics of organic materials, and extensive research is started on applications of an organic electroluminescent display element having excellent characteristics such as DC low-voltage driving, a thin size, luminescence, etc. to a color display.
The organic electroluminescent element (referred to as an "organic EL element" hereinafter) has the ideal characteristics as a self-luminescent display device that it has a thickness of 1 micro meters or less, and converts electrical energy into optical energy to emit light in a plane form by injecting an electric current.
FIG. 13 shows an example of a conventional organic EL element 10. The organic EL element 10 comprises an ITO (Indium tin oxide) transparent electrode 5, a hole transport layer 4, a luminescent layer 3, an electron transport layer 2, and a cathode (for example, an aluminum electrode ) 1, which are deposited on a transparent substrate (for example, a glass substrate) 6 in this order, by, for example, a vacuum deposition method.
When a DC voltage 7 is applied between the transparent electrode 5 as an anode and the cathode 1, holes as the carriers injected from the transparent electrode 5 and the electrons injected from the cathode 1 reach the light emitting layer 3 through the electron transport layer 2 and the hole transport layer 4 to produce recombination of the electrons and holes to emit light 8 with a predetermined wavelength. This can be observed from the back of the transparent substrate 6.
A zinc complex can be contained in the luminescent layer 3. For example, the luminescent layer 3 may comprise substantially only a zinc complex (however, a plurality of zinc complexes may be used), or the layer 3 may contain a zinc complex and a fluorescent material. Alternatively, a zinc complex and another luminescent material such as anthracene, naphthalene, phenanthrene, pyrene, chrysene, perylene, butadiene, coumarin, acridine, stilbene, or the like may be combined. Such a zinc complex or a mixture of a zinc complex and a fluorescent material can also be contained in the electron transport layer 2.
FIG. 14 shows an organic EL element 20 as another conventional example in which the luminescent layer 3 is omitted, and a zinc complex or a mixture of the zinc complex and a fluorescent material is contained in the electron transport layer 2 so as to emit light 18 having a predetermined wavelength from the interface between the electron transport layer 2 and the hole transport layer 4.
FIG. 15 shows an illustrative example of the organic EL display. In FIG. 15, organic layers (the hole transport layer 4 and the light emitting layer 3 or the electron transport layer 2) are provided between the cathode 1 and the anode 5 which are provided in stripes crossing to form a matrix, so that light is emitted from many intersections, or pixels, when a signal voltage is applied from a luminance signal circuit 30 and a control circuit 31 containing a shift register.
The organic EL display configured as described above can thus be used as not only a display but also an image reproducing device. The above stripe pattern is provided for each of red (R), green (G) and blue (B) colors so that the display can be used for full color or multi-color display.
In a display device comprising the organic EL element having a plurality of pixels, the luminescent organic thin film layers 2, 3 and 4 are generally held between the transparent electrode 5 and the metallic electrode 1. Light is emitted from the back of the transparent electrode 5.
However, the above-mentioned organic EL element still has unsolved problems.
For example, in the application of the organic EL element to a color display, it is necessary and essential to emit light of the primary colors R, G and B with stability. However, in the present state, the materials capable of emitting red and blue which can be applied to a display and which have sufficient stability, chromaticity, luminance. The red and blue materials are being studied in various fields. An aluminum-quinoline complex promising as a green luminescent material, but this material has a slight deviation of chromaticity.
In consideration of application to a full-color display, low-voltage driving is necessary and essential, and there is still room for improvement in each of the primary colors R, G and B.
Accordingly, there is a need for an improved optical element and method of manufacturing an optical element capable of emitting light with high luminance with low voltage driving requirements and organic layers with improved stability.